The present invention relates to a vehicle speed control apparatus for controlling the speed of an own vehicle relative to a preceding vehicle.
A variety of devices have been developed to alleviate burdens on the driver of a running vehicle and to enhance driving safety. Illustratively, cruise control systems and like apparatuses for controlling vehicles running at a constant speed have already been commercialized. Currently under development are driving control apparatuses that measure by radar a headway distance and a relative speed of a host vehicle with respect to a preceding vehicle so as to let the host vehicle run while maintaining the headway distance or avoid collision with the preceding vehicle. A cruise control system, once its cruise mode switch is turned on, allows the vehicle to maintain its speed until an operation such as the stepping on the brake pedal is carried out to cancel the cruise mode.
One conventional headway distance maintaining system allows the host vehicle to maintain the headway distance if there is a preceding vehicle, and activates a following control device to get into cruise mode based on an established speed if there is no preceding vehicle. Japanese Patent Laid-open No. Hei 8-192661 discloses a driving control apparatus claimed to work as follows: if the host vehicle trailing the preceding vehicle loses the latter from a radar range, the apparatus sets as a target vehicle speed the vehicle speed in effect when the preceding vehicle was lost. Thereafter, if a suitable reset operation is performed during a predetermined period of time, an initially set vehicle speed is restored; if no reset operation is carried out, the target vehicle speed is established as a new vehicle speed.
One disadvantage of the above and other similar conventional apparatuses is that if the preceding vehicle entering a curve is captured by radar, the apparatus causes the host vehicle to start decelerating well before the curve is reached so as to keep the headway distance unchanged. Another disadvantage is that when the preceding vehicle starts accelerating at the exit of a curve, the host vehicle behind it also starts accelerating even if the trailing vehicle is still negotiating the curve.
The present invention has been made in view of the above circumstances and provides a vehicle speed control apparatus allowing a host vehicle trailing a preceding vehicle along a curve to accelerate and decelerate in accordance with the configuration of the road.
In carrying out the invention, there is provided a vehicle speed control apparatus comprising detecting means for detecting a headway distance, a relative speed and a direction of a host vehicle with respect to a preceding vehicle, and computing means for computing a speed of the preceding vehicle based on an own speed and the relative speed of the host vehicle. After the speed of the preceding vehicle transiting a given point on a road is acquired and retained, the host vehicle is controlled to pass that point at a speed not exceeding the retained speed of the preceding vehicle. When passing the point in question, the host vehicle may be controlled to run at a speed not exceeding the retained speed of the preceding vehicle while keeping the headway distance to the preceding vehicle from getting shorter than a predetermined value. If subject to a yaw rate of a predetermined minimum value, the host vehicle may be controlled to keep its own speed from exceeding a maximum speed determined by the yaw rate. If the vehicle speed control apparatus further comprises detecting means for detecting headway distances and directions of the host vehicle with respect to a plurality of preceding vehicles, and if the plurality of preceding vehicles change their courses in the same direction, the apparatus may detect the presence of a curve ahead of the host vehicle and may keep the speed of the host vehicle entering the curve not in excess of the speeds of the preceding vehicles. If the plurality of preceding vehicles change their courses in different directions, the vehicle speed control apparatus may control the speed of the host vehicle by getting road information from an onboard navigation device to determine whether the directional change is attributable to the preceding vehicles heading into road branches or changing lanes.
The inventive vehicle speed control apparatus above controls the host vehicle entering a curve in a way that keeps its speed from exceeding the speed of the preceding vehicle that entered the curve earlier. This provides speed control of the host vehicle in keeping with the configuration of the road. Because the lateral acceleration of the host vehicle is proportional to the product of the own speed and a yaw rate of the vehicle, the maximum vehicle speed may be set in accordance with the yaw rate. This keeps the lateral acceleration of the host vehicle from exceeding a predetermined level, thus enhancing ride quality in keeping with the road configuration. When the host vehicle is running a multiple-lane road, the inventive apparatus may detect a plurality of preceding vehicles changing their courses to ascertain the presence of curves and thereby allow the host vehicle to travel in accordance with the road configuration.